Recently, in response to the development of a variety of technologies such as Bluetooth, in-vehicle sound solutions (e.g. a voice call solution) have become convenient and are being actively used. However, the call quality of such solutions has not reached the level of call quality obtained from using a typical mobile phone and their environment is inferior, since there are several problems such as noise inside a driving vehicle and echo caused by the use of a speaker. In addition, as voice recognition becomes more common, it is expected that a speech recognition success rate of considerable level be guaranteed inside the vehicle.
For in-vehicle calling and speech recognition, a voice signal must be inputted first using a microphone. In this case, when the voice signal is inputted using only a single microphone, a sufficient signal noise ratio (SNR) of the signal is not ensured. In addition, the voice signal is very vulnerable to acoustic interference, such as driving noise and distortion and echo caused by the space of the vehicle, which is problematic.
In addition, a sound solution for in-vehicle calling or speech recognition is required to receive voices of the driver as well as voices of other persons. As for this problem, the input SNR can be improved and made robust against sound interference signals by forming a beam using a microphone array.
As an approach for forming such a beam, techniques for an adaptive beamformer are disclosed. Among these techniques, a linearly constrained minimum variance (LCMV) adaptive beamformer was disclosed in the report of Otis Lamont Frost III in 1972.
The adaptive beamformer is frequently used for in-vehicle sound solutions. The adaptive beamformer is generally used to provide a more efficiently sound solution inside a vehicle by adaptively changing the direction of a beam in response to a sound source (e.g. a speaker) or a noise.
However, even though the beam is formed using the adaptive beamformer of the related art, there is a problem in that it is difficult for the microphone array using one beam to receive voices of several persons or the performance thereof is low.
For this, a microphone array system having two or more beams can be required. Since various noises and interference signals are present, the difference in steering between an interference signal and a desired signal can also be decreased, thereby requiring beamforming to be more precise.